Boil the pasta: Bring a large pot of water to a boil over high heat.

While the water heats, finely chop the onion and garlic, roughly chop the walnuts, finely chop the parsley, and zest and juice the lemon.

When the water boils, season it with enough salt to taste lightly salty. Add the penne to the pot and cook it until tender with a little firmness in the center, following the package time as a guide.

Start the sauce: Set a large skillet over medium heat. Add the chopped walnuts to the dry skillet and toast them for 2 to 3 minutes, stirring often, until they smell nutty and look a shade darker.

Transfer the toasted walnuts to a plate so they do not keep cooking in the hot skillet.

Pour the olive oil into the same skillet and keep it over medium heat.

Add the chopped onion and 1/2 teaspoon of the kosher salt to the skillet. Cook for 4 to 5 minutes, stirring now and then, until the onion is soft and lightly golden at the edges.

Add the garlic to the skillet. Cook for about 30 seconds, stirring, just until it smells fragrant.

Scoop 1/2 cup of the starchy pasta water from the pot into the skillet. Add the frozen peas to the skillet and stir them into the onion mixture.

Cook the peas for 2 to 3 minutes, stirring now and then, until they are hot and bright green.

Use a potato masher or the back of a sturdy spoon to mash about half of the peas in the skillet. The mixture should look chunky and a little creamy, not completely smooth.

Finish the pasta: Reserve 1 cup more of the pasta water from the pot. Drain the penne in a colander right when it is ready.